1. Technical Field
The following description relates to one or more techniques to correct an image forming property of an image forming device.
2. Related Art
An image forming device such as a color laser printer has been known, which includes a plurality of image forming units aligned along a sheet carrying belt such that toner images of respective different colors are sequentially transferred onto a sheet being conveyed on the sheet carrying belt by the image forming units. In such an image forming device, when the respective toner images are transferred into different positions on the sheet by the image forming units, a formed image becomes a low-quality one.
In order to secure the quality of the image, a technique referred to as registration to correct positional deviations between the toner images transferred onto the sheet has been employed. According to such a correction technique, a pattern including a plurality of marks is formed on a surface of the sheet carrying belt by each image forming unit, and the positional deviations between different color toner images are determined by detecting locations of the marks with an optical sensor. Then, based upon a result of the detection, the positional deviations between the toner images are corrected. It is noted that the measured deviations may periodically fluctuate due to unevenness in thickness of the sheet carrying belt. Therefore, in general, the patterns are formed throughout a circuit of the sheet carrying belt. The positional deviations of the marks included in the pattern are measured in a plurality of locations on the sheet carrying belt, and the positional deviation correction is performed based upon an average value of the positional deviations measured.
However, the sheet carrying belt has an region thereon in which an actual positional deviation is significantly different from the average value of the positional deviations determined based upon the patterns formed throughout an entire circumference of the sheet carrying belt. Therefore, when the average value throughout the entire circumference of the sheet carrying belt is utilized in the case where a sheet is conveyed in the region, the image quality is deteriorated. In order to overcome this problem, an image forming device has been proposed, in which a region on the belt to be used for carrying a sheet is searched, and the positional deviation correction is performed using an average value of positional deviations determined based upon the pattern formed within the searched region (see Japanese Patent Provisional Publication No. 2002-91115).